An automobile is equipped with seatbelts and airbags as devices for securing the safety of occupants. In recent years, in order to further improve the performance of a seatbelt and an airbag, there has been a tendency to control the movement of such safety facilities in conformity with an occupant's weight (body weight). For example, the expansion gas volume and expansion speed of an airbag and the pretensioning of a seatbelt are adjusted in conformity with an occupant's weight. For that purpose, it is necessary to know the weight of an occupant in a seat by some sort of means. As an example of such means, a means of disposing load sensors (load cells) at the four corners of a seat rail assembly, adding up the loads in the vertical direction imposed on the load cells, and by so doing measuring the weight of a seat including an occupant's weight has been proposed (Japanese Unexamined Patent Publication No. H11-304579).
With regard to a mechanical quantity sensor for detecting load, pressure, etc., various sensors has been proposed in accordance with the kind of a substrate and the kind of a strain sensitive material used for a resistive element. Typical proposed examples are: (1) a sensor produced by using a film comprising a resin such as polyester, epoxy, polyimide or the like as a substrate and forming on the surface of the substrate a lamellar resistive element comprising Cu—Ni alloy, Ni—Cr alloy or the like by vapor deposition or sputtering, (2) a sensor produced by using a glass plate instead of the aforementioned resin film (Japanese Examined Patent Publication No. H3-20682), and (3) a sensor produced by using a metal base material the surface of which is covered with a crystallized glass layer as a substrate and forming a resistive element on the surface thereof by coating it with paste and baking it (Japanese Unexamined Patent Publication No. H5-93659).
The magnitude of a mechanical quantity is measured in the following way. When a force or a load is imposed on a mechanical quantity sensor from outside, a resistive element formed on the surface of a substrate deforms together with the substrate. The imposed mechanical quantity is detected by measuring the change of an electric resistance between a pair of electrodes formed by connecting the resistive element, the change of the electric resistance being caused by the change of the length and sectional area of the resistive element. A mechanical quantity sensor that uses a metal base material on the surface of which a crystallized glass layer is formed as a substrate is most suitable as a sensor used under a harsh environment because, unlike other types of sensors, each of the component elements interdiffuses between the metal base material and the crystallized glass layer and also between the crystallized glass layer and a resistive element, and thus the adhesiveness between them is very strong. As a resistive element of a mechanical quantity sensor of this type, an element formed by being coated with resistive paste containing ruthenium oxide that functions as a resistive material, then dried and baked is known.
As metal base materials used for mechanical quantity sensors, a vitreous enamel steel, a stainless steel, a silicon steel, various alloy materials such as nickel-chromium-iron, nickel-iron, Kovar, Invar, etc., clad materials of those alloy materials and the like can be selected. Japanese Unexamined Patent Publication No. 2000-180255 discloses a technology that uses a stainless steel sheet as a metal base material. Japanese Unexamined Patent Publication No. H10-38733 discloses a technology that uses SUS 430 as a metal base material from the viewpoint of the adhesiveness with an insulating glass layer. Japanese Unexamined Patent Publication No. H5-93659 discloses a technology that uses SUS 430 concretely as a metal base material from the viewpoint of the necessity of coordinating the expansion coefficient thereof with that of a glass layer.
However, with a metal base material based on the aforementioned existing technologies, glass adhesiveness and high temperature oxidation resistance during baking are insufficient and therefore the metal base material has not been put into practical use. It is preferable that a sensor substrate is made of a stainless steel sheet and that an insulating glass layer and the layers of a resistive element and electrodes are solidified by baking (the schematic illustration is shown in FIG. 1). In this light, a stainless steel that has a high thermal resistance and an excellent glass adhesiveness so that sensor members may be baked together when each of the layers is baked at a high temperature has strongly been longed for.
When a crystallized glass layer functioning as an insulating layer, a strain sensitive resistive element and electrodes are baked and resultantly solidified in the form of layers onto a stainless steel sheet functioning as the substrate of a sensor, it is necessary to coordinate the linear expansion coefficients of a metal base material and a glass layer with each other in order to improve the adhesiveness between them. Since the baking is applied at a temperature of 900° C. or lower, it is necessary for the linear expansion coefficients of the metal base material and the glass layer to approximate each other, not only in the vicinity of room temperature but also in the temperature range from 20° C. to 900° C. If the difference in the average linear expansion coefficient is large between a metal base material and a crystallized glass layer, the adhesiveness between them deteriorates considerably and therefore they do not function as the substrate of a resistive element. Whereas the average linear expansion coefficient of generally used crystallized glass is 13 to 16×10−6/° C., that of a conventionally used stainless steel is about 13×10−6/° C. Accordingly, the difference in the average linear expansion coefficient is too large between the stainless steel substrate and the glass layer, and thus sufficient glass adhesiveness cannot be obtained.